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Author Topic: Door Knob Touch Alarm  (Read 2476 times)
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Version 1.0  If the door knob is touched (from the other side of the door) the alarm goes off.

I wired a 10 uf capacitor in series with a 1 mega-ohm resistor from 5v to Analog0 like this:

The Arduino is hanging on the doorknob by the loop in the wire.  AnalogRead records the change in values.  If the wire is touched directly then analogRead drops quite a bit, if the doorknob is touched on the other side of the door, it drops just a little bit.  
I also wired a potentiometer so that the sensitivity can be adjusted and the alarm will be activated by that small drop in value.

//When someone touches the doorknob the analogRead value from the wire drops a bit.

int sensorPin = 0;  
int ledPin = 13;      
int sensorValue = 0;  
int pot;
int sensor;

void setup() {
  pinMode(ledPin, OUTPUT);  

void loop() {
  sensorValue = analogRead(sensorPin); //the doorknob sensor wire
pot=analogRead(5);  // the potentiometer is connected to analog 5

 Serial.print(sensorValue);  //sensor wire value
 Serial.print("      ");
 Serial.println(pot);        //potentiometer value

 if(sensorValue<pot){   // this allows you to turn the potentiometer to a sensitive borderline value
                       // when someone touches the knob it dips just below the pot value

     digitalWrite(13,HIGH); //the led (alarm) lights up.  This could be replaced by a buzzer

For the next version:  
I don't have a wide variety of capacitors and resistors to experiment with so there may be a better combination.  
Also, the LED could be replaced by a buzzer.

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